“But NOLOCK won’t hurt me if I’m using an index!”

A common misconception that NOLOCK is safe as long as a query uses an index seek is debunked with a live demo. A transaction flips row values while a concurrent NOLOCK query with an index hint runs, and the NOLOCK query returns thousands of impossible rows (values that should never coexist). The explanation: SQL Server executes the seek plus key lookup in two steps, and NOLOCK ignores in-flight locks, so it can read a row's index value before it's updated and its clustered index value after it's updated, producing inconsistent results. Making the index covering avoids the specific bug but isn't practical for every query. The conclusion: NOLOCK is only acceptable when result accuracy genuinely doesn't matter, such as some executive dashboard reporting.

Nguồn: https://www.brentozar.com/archive/2026/09/but-nolock-wont-hurt-me-if-im-using-an-index. 8 Sync News chỉ tóm tắt và dẫn link; bản quyền nội dung thuộc tác giả và nguồn gốc.